I. Field of the Invention.
This invention relates to the manufacture of holographic optical elements, and in particular to the manufacture of holographic filters of the type used in connection with the spectral sorting and classification of radio frequency signals and the detection and removal of noise and unwanted frequencies from those signals.
II. Description of Related Art.
1. Uses for Holographic Filters.
Holographic filters are useful in a variety of applications involving analysis of information-carrying beams of light. The application described below involves use of holographic filters to analyze signals of interest in the radio frequency band.
In the field of radio frequency signal processing, it is known to modulate the signal of interest onto a carrier laser beam for subsequent processing. Spectral sorting and filtering of the beam can then be accomplished at essentially the speed of light by optical techniques.
Acousto-optic modulator devices for modulating the radio frequency signals onto the carrier laser beam are well known. Such modulators work by transforming a radio frequency signal into an acoustic wave which modulates the refractive index of an optical material, through the use of a piezo-electric crystal transducer causing compressions and rarefactions in the medium that are periodic with the radio frequency signal.
The modulated medium appears to an incident laser beam as a dynamic phase grating which shifts the frequency of the carrier by an amount proportional to the radio frequency of the signal of interest. Such an acousto-optic modulator is described in U.S. Pat. No. 4,699,466. Other types of acousto-optic modulators are also known to those skilled in the art.
The resultant modulated carrier beam is then Fourier transformed using an appropriate lens such as the lens described in U.S. Pat. No. 4,421,379. The lens described in U.S. Pat. No. 4,421,379 converts the modulated carrier beam into the Fourier frequency domain. The reason that the modulated beam is transformed into the frequency domain is that the Fourier transform of a signal is essentially a model of the signal waveform spectra over all time, and therefore permits evaluation of every signal frequency that may be present in the beam.
Once the carrier beam has been modulated together with the radio frequency signal of interest, and converted into the Fourier frequency domain, the radio frequency signal may then be analyzed for the presence of particular frequencies. For this purpose, a holographic filter is used which separates the frequencies of interest from the remainder of the Fourier separated frequencies, as illustrated in FIG. 1.
The holographic filter 2 shown in FIG. 1 is essentially a set of interference fringe patterns 1 having a line spacing S. Using principles of diffraction, only components of those frequencies fl to fn in the signal of interest which coincide with a unique fringe pattern in the filter, i.e., the correlated spectra 3, will be diffracted. The remainder of the laser beam passes through the filter as uncorrelated spectra 4 without diffraction. The uncorrelated spectra 4 also contains the zero order of grating 2.
Once the optically modulated laser beam has been correlated by selectively diffracting specific frequencies in the signal of interest, the correlated spectrum may then be analyzed, recorded, or monitored. Also, a "correlation function" for the signal of interest may be developed. The correlated spectrum may, finally, be converted back to the time domain by an inverse Fourier transform, and subsequently demodulated to recover the original signal of interest for retransmission or additional data processsing.
Although one known use of a holographic filter has been described above, it will be recognized that holographic filters may have other applications, such as optical beam switching and in the fields of image processing or character recognition.
2. Disadvantages of Conventional Holographic Filters.
The primary disadvanatage of holographic filters is the complexity of creating holographic recordings for use in the filters. Formation of a hologram has previously been understood to require the interference of two coherent light beams, and the optical recording of the resultant interference pattern.
Such a system is described, for example, in U.S. Pat. No. 4,597,630. The system described in U.S. Pat. No. 4,597,630 splits a single coherent light beam such as a laser beam which has been phase modulated with a radio frequency signal using an acousto-optic modulator. One of the beams is used as the signal-carrying object beam and the other beam is a self-derived reference beam. The two beams are caused to interfere and the resultant interference pattern is recorded by a photo-optical element or medium.
While the system of U.S. Pat. No. 4,597,630 has proved successful, the difficulties inherent in the process are illustrated by the conditions necessary to create a recordable interference pattern. First, both the object and reference beams must have the same frequency and wavelength. Second, the phase difference between the waves at a given point must not vary in time. Third, the relative amplitude of the two signals must be fixed. Fourth, the light signals must have the same polarization. Finally, the path difference between the signals must be within the coherent length of the source laser beam.
The system of U.S. Pat. No. 4,597,630 clearly meets these conditions, but it would nevertheless be advantageous to create holographic filters which are suitable for applications such as the above-described RF spectra correlation and which do not require the meeting of these conditions.